The invention generally relates to fans, particularly those used to move air through radiators and heat exchangers, for example, in vehicle engine-cooling assemblies.
Typical automotive cooling assemblies include a fan, an electric motor, and a shroud, with a radiator/condenser (heat exchanger), which is often positioned upstream of the fan. The fan comprises a centrally located hub driven by a rotating shaft, a plurality of blades, and a radially outer ring or band. Each blade is attached by its root to the hub and extends in a substantially radial direction to its tip, where it is attached to the band. Furthermore, each blade is xe2x80x9cpitchedxe2x80x9d at an angle to the plane of fan rotation to generate an axial airflow through the cooling assembly as the fan rotates. The shroud has a plenum which directs the flow of air from the heat exchanger(s) to the fan and which surrounds the fan at the rotating band with minimum clearances (consistent with manufacturing tolerances) so as to minimize recirculating flow. It is also known to place the heat exchangers on the downstream (high pressure) side of the fan, or on both the upstream and downstream side of the fan.
Like most air-moving devices, the axial flow fan used in this assembly is designed primarily to satisfy two criteria. First, it must operate efficiently, delivering a large flow of air against the resistance of the heat exchanger and the vehicle engine compartment while absorbing a minimum amount of mechanical/electrical power. Second, it should operate while producing as little noise and vibration as possible. Other criteria are also considered. For example, the fan must be able structurally to withstand the aerodynamic and centrifugal loads experienced during operation. An additional issue faced by the designer is that of available space. The cooling assembly must operate in the confines of the vehicle engine compartment, typically with severe constraints on shroud and fan dimensions.
To satisfy these criteria, the designer must optimize several design parameters. These include fan diameter (typically constrained by available space), rotational speed (also usually constrained), hub diameter, the number of blades, as well as various details of blade shape. Fan blades are known to have airfoil-type sections with pitch, chord length, camber, and thickness chosen to suit specific applications, and to be either purely radial in planform, or swept (skewed) back or forward. Furthermore, the blades may be symmetrically or non-symmetrically spaced about the hub.
By controlling blade pitch as a function of radius, we have discovered a fan blade design for a banded fan which is adapted to the flow environment created by a heat exchanger and shroud, and which hence provides greater efficiency and reduced noise. Blade pitch directly affects the pumping capacity of a fan. It must be selected based on the rotational speed of the fan, the air flow rate through the fan, and the desired pressure rise to be generated by the fan. Of particular concern is the precise radial variation of pitch, which depends on the blade skew and also on the radial distribution of airflow through the fan.
Skewing the blades of a fan (often done to reduce noise) changes its aerodynamic performance and hence blade pitch must be adjusted to compensate. Specifically, a blade that is skewed backward relative to the direction of rotation generally should have a reduced pitch angle to produce the same lift at a given operating condition as an unskewed blade that is in all other respects the same. Conversely, a forwardly skewed fan blade generally should have increased pitch to provide equal performance. The invention takes these factors into account.
In addition the invention accounts for radial variation in air inflow velocity. In the case of the assembly shown in FIG. 1, the incoming air passes through the radiator and is then forced by the shroud plenum to converge rapidly from the large cross-sectional flow area of the radiator to the smaller flow area of the fan opening in the shroud. This results in a flow field at the fan that is highly non-uniform radially.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.